This invention relates to an engine control device capable of reducing the sulfur content in exhaust gases from an engine.
One of well-known techniques for reducing nitrogen oxides (NOx) contained in engine exhaust gases is exhaust gas recirculation (EGR), in which part of the exhaust gas is returned to an intake air line as described in Japanese Unexamined Patent Publication No. 6-229322, for instance.
In an engine in which the air-fuel ratio is increased to produce a lean air-fuel mixture in a particular operating range, it is a common practice to provide an NOx-absorbing material which absorbs NOx contained in exhaust gas emissions when the air-fuel ratio is high and releases the NOx when the concentration of oxygen in the exhaust gas has dropped so that the released NOx is converted through a reduction reaction. As an example, this approach is used in a direct injection engine whose injector injects fuel directly into a combustion chamber during a compression stroke to produce stratified charge combustion in a low-load range. The NOx-absorbing material of the aforementioned type has such properties that it absorbs sulfur oxides (SOx) more easily than the NOx in the exhaust gas emissions when fuel or engine oil containing sulfur constituents produced in particular areas is used. The NOx-absorbing ability of the NOx-absorbing material is considerably degraded once it has been poisoned by absorbing the SOx.
Japanese Unexamined Patent Publication No. 6-066129 discloses a technique for overcoming such poisoning of an NOx-absorbing catalyst by SOx by increasing catalyst temperature when the concentration of sulfur in the NOx-absorbing catalyst is high. This technique uses an electric heater for increasing the catalyst temperature.
Another approach to the solution of the SOx poisoning problem is disclosed in Japanese Unexamined Patent Publication No. 11-107740, in which an NOx-absorbing catalyst is provided in an exhaust passage of a direct injection lean-burn gasoline engine, and the fuel is injected twice during a period from an intake stroke to a succeeding compression stroke to produce exhaust gas which is rich in carbon monoxide (CO) to thereby eliminate sulfur deposited in the NOx-absorbing catalyst.
As stated above, the technique disclosed in Japanese Unexamined Patent Publication No. 6-066129 requires the electric heater for increasing the catalyst temperature. This approach is not desirable from the viewpoint of space and cost savings. On the other hand, the approach of Japanese Unexamined Patent Publication No. 11-107740 makes it necessary to increase the catalyst temperature even more effectively, which is not desirable either.